Abstract Memory CD8 T cells play a critical role in mediating protection against infection by intracellular pathogens or in anti-tumor responses. Since durable protective immunity is the goal of vaccination, understanding the mechanisms that regulate nave-to-memory CD8 T cell differentiation and long-term maintenance of CD8 T cell memory is critical for rational design of vaccines. Technical advances have allowed tracking of some, but not all, CD8 T cells responding to infection, and a body of information now exists describing phenotypic/functional/transcriptional/epigenetic changes that occur in CD8 T cells of known Ag-specificity during their activation, expansion, and memory generation in mice. However, current mouse models of infections may underrepresent the complexity of human immune responses. We recently described a technique that can be used to track pathogen-specific CD8 T cell responses in any mouse and noted substantial discord in the magnitude/kinetics/memory CD8 T cell differentiation in individual outbred mice following infection. Therefore, one can argue that given the variability of immune responses and clinical outcomes generally observed in human population, limited number of inbred mouse models cannot fully recapitulate the range of immune phenotypes expressed across diverse humans. Collaborative Cross (CC) mouse model could represent an exciting approach to examine the biological networks and genetic factors that govern divergent CD8 T cell outcomes following infection. Commercially available CC lines, derived using a funnel breeding strategy with eight founder strains, contain defined single nucleotide polymorphisms and insertions or deletions that results in vast genetic diversity between lines. This is similar to outbred cohorts, however, the progeny within established lines are inbred allowing for precise analyses, reproducibility and comparative studies. Thus, the goal of this proposal is to start decoding variability of pathogen-specific memory CD8 T cell differentiation observed in genetically distinct hosts using CC lines. Proposal is based on exciting preliminary data that identified CC lines in which memory CD8 T cell development deviates from `canonical' memory CD8 T cell responses observed in inbred B6 mice after viral infection. We will test the overall hypothesis that nave-to-memory CD8 T cell differentiation to infection is complex and potentially controlled by host specific biological and/or genetic factors. Aims proposed are: SA1 Define factors controlling differentiation of memory CD8 T cells in defined strains of CC mice; SA2 Identify genetically variant genome regions associated with variance in memory CD8 T cell differentiation.